1. Field of the Invention
The present invention relates to a high fidelity audio amplifier with low distortion and very high efficiency, the amplifier comprising:                an output for feeding a load;        a voltage generator presenting very high linearity and very low output impedance;        a current source having its output connected to the output of the voltage generator at a coupling point connected to the output, the output of the voltage generator being connected directly to the coupling point without any resistive element being interposed between them; and        a control stage for controlling the current source from the current delivered by the voltage generator.        
2. Description of the Related Art
Distortion and efficiency are characteristics that are important for an audio amplifier.
It is known that the distortion of a linear voltage amplifier is relatively low. An amplifier of that type is referred to as “class A” or “class AB”. Such a structure is used for very up-market amplifiers. Nevertheless, the efficiency of such amplifiers is poor, being of the order of 25% to 70%.
Conversely, amplifiers of class D, using pulse-width modulation (PWM) have very good efficiency. They are constituted by so-called digital amplifiers making use of switch elements. Efficiency lies in the range 95% to 98%.
Such amplifiers are not very linear since they make use of inductors and they thus present a high level of distortion leading to audio performance that is mediocre.
In an attempt to combine the advantages of those two types of amplifier, i.e. to obtain good efficiency with low distortion, it is known to couple a class A amplifier with a class D amplifier so as to form a class AD amplifier.
Thus, a voltage amplifier presenting low output impedance and low distortion factor is coupled with a current amplifier having high efficiency.
One such solution is described in U.S. Pat. No. 6,407,619, for example. That patent describes coupling a voltage generator with a current generator, the current generator operating in an open loop outside the negative feedback loop of the voltage generator.
The current generator is servo-controlled from a measurement of the total current delivered by both generators, such that the current delivered by the current generator is 5 to 20 times greater than that delivered by the voltage generator. The coupling between the two generators is direct in order to reduce the distortion of the system.
The distortion of the amplifier described is low, but its efficiency is not very high.
This limited efficiency can be explained in particular because the voltage generator does not present high efficiency in order to be able to obtain low output impedance, and also by the presence of resistance in the circuit connecting the load to ground, which leads to a phase shift in the current when the load is not purely resistive, as is the case of a loudspeaker. This leads to a degradation in the damping factor of the amplifier.
Since the current generator is operating in an open loop, its gain is relatively inaccurate, and furthermore the phase shift in the current measurement implies that the current generator must present gain that is significantly lower than 1 in order to ensure that the system is stable. The gain limitation restricts the current ratio between the two generators to values lying in the range 5 to 20.
Finally, the relatively high resistance resistor connected in series with the current generator greatly degrades efficiency.